Re: [RFC PATCH 2/3] sched: add yield_to function

Srivatsa Vaddagiri
2010-12-10T08:39:06Z

On Thu, Dec 09, 2010 at 11:34:46PM -0500, Rik van Riel wrote:
> On 12/03/2010 09:06 AM, Srivatsa Vaddagiri wrote:
> >On Fri, Dec 03, 2010 at 03:03:30PM +0100, Peter Zijlstra wrote:
> >>No, because they do receive service (they spend some time spinning
> >>before being interrupted), so the respective vruntimes will increase, at
> >>some point they'll pass B0 and it'll get scheduled.
> >
> >Is that sufficient to ensure that B0 receives its fair share (1/3 cpu in this
> >case)?
>
> I have a rough idea for a simpler way to ensure
> fairness.
>
> At yield_to time, we could track in the runqueue
> structure that a task received CPU time (and on
> the other runqueue that a task donated CPU time).
>
> The balancer can count time-given-to CPUs as
> busier, and donated-time CPUs as less busy,
> moving tasks away in the unlikely event that
> the same task gets keeping CPU time given to
> it.

I think just capping donation (either on send side or receive side) may be more
simpler here than to mess with load balancer logic.

> Conversely, it can move other tasks onto CPUs
> that have tasks on them that cannot make progress
> right now and are just donating their CPU time.
>
> Most of the time the time-given and time-received
> should balance out and there should be little to
> no influence on the load balancer. This code would
> just be there to deal with exceptional circumstances,
> to avoid the theoretical worst case people have
> described.

- vatsa